[CBN] Refactor subscription size flow

[arosiclair]

• HELD on [CBN] Set new best practices #79039

Let's refactor the subscription size flow(Settings > Subscription > Subscription Settings > Subscription Size) to use useSubPage.

https://docs.google.com/document/d/17dsyKl3G3jPJHi1ymJMnxckQ8_bM_A07TKHcO7w5djk/edit?tab=t.0#heading=h.rerhlropuyit

[PiyushChandra17]

I'd like to work on this, if it's open for external contributors. Thanks!

[arosiclair]

Held

[bernhardoj]

@dukenv0307 PR is ready